1. Field of the Invention
This invention relates, in general, to electronic color printers and, more specifically, to apparatus and methods of storing page data in memory buffers of electronic color printing devices.
2. Description of the Prior Art
Color printers capable of printing pages with text, graphics, and image information can have very demanding memory requirements. In order to obtain the highest quality text and line graphics printing, the data transferred to the printhead must contain all of the necessary information to make the printed data visually accurate. For text and line graphics data, this can be in the form of direct, high resolution "bit-mapped" data with several bits of data associated with each printed pixel. However, for area fill graphics and images, the need for a wider color gamut than the direct bit-mapping system can provide is desirable. This is because more colors can be produced by using halftoning for the rendition of filled area and images.
The conventional approach is to fill a printing buffer using a general purpose CPU to render the full page in memory, and then to print it. To fully utilize a multi-bit gray-level printhead in this fashion, four page buffers would be required, one for each color separation of the colors black, cyan, magenta, and yellow. A gray-level printhead which can have a four-bit gray scale per pixel for each of the color separations would require a tremendous amount of memory for each page in the buffer. At 400 dots per inch (dpi), a full 11".times.17" page would require approximately 64 Mbytes. Such a size is prohibitive, and it is desirable to store the information in a much smaller memory area without materially detracting from the quality of the finished product.
There are three requirements that need to be met in the design of a data structure capable of Providing the memory space needed to economically store multi-bit, gray-level, color page data. Along with maintaining the memory at the smallest size possible consistent with maintaining an acceptable level of image quality, it is desirable that the memory or buffer system be expandable from a binary system to a multi-bit printing system. In addition, a desirable implementation of a page buffer memory system would be one in which the data structure keeps the hardware costs to a minimum.
In order to obtain a more efficient memory structure for the page buffer of a color printer, it is important to recognize that the sharpness or resolution of printed information must be stored more precisely than color information so that the printed page will be perceived by an observer as having full memory storage. In other words, it is possible to sacrifice some of the resolution defining the color of the printed data without that sacrifice being perceived by the observer. On the other hand, sharp contrast areas such as text and line graphics require more precise data storage and cannot tolerate the sacrifices acceptable in storing color information. The invention disclosed herein uses these principles.
The cross-referenced application, Ser. No. 07/353,715, describes two patents which are known to be relative to the concepts taught by this invention. The cross-referenced application itself describes a system for data storage which overcomes some of the problems encountered according to the Prior art. However, it is desirable, and it is an object of the present invention, to be able to more accurately store in memory the image information which will be used to ultimately print the output page.
In some images, optimum print quality cannot be obtained by using a single luminance level for an entire 16-pixel array, as is taught by the referenced patent application. The buffer system of the present invention adequately provides for variations in luminance levels throughout the pixel cell.
Another problem associated with the prior art is the handling of a transition area within the boundaries of the pixel cell. This can occur where text or line graphics begins adjacent to image or area fill graphics. The portion of the pixel cell which is high contrast text or lines needs to be printed in a solid color whereas the portion which is image or area filled would often be halftoned. However, since the pixel cell can only be specified for one type of representation according to the prior art, a mixture of solid and halftones within the same pixel cell cannot be realized. As a result, solid pixels are often printed near the border of text and lines in areas adjacent to halftoned pixels. The result can be an objectionable staircase effect along the edges of the lines. It is, therefore, desirable and another object of this invention to provide a system whereby transitions in types within the pixel cell are adequately handled.
Although it is possible to produce black from a process of printing three colors, some black images or graphics are easier to represent in the actual operating environment with only black toner. This additional information about black printing must be stored in the page buffer to be of use to the output device or printer. Thus, the present invention also provides the desirable capability of storing in the page buffer data concerning the type of process to be used in reproducing black in the output page.